This invention relates to a novel method of making microcapsules and particularly to a method of making microcapsules having an improved permeability which are useful for the manufacture of pressure-sensitive copying papers or heat-sensitive recording papers.
Pressure-sensitive copying papers and heat-sensitive recording papers which utilize the color developing reaction between electron donating organic chromogenic material (hereinafter referred to as "color former") and electron accepting acidic reactant material (hereinafter referred to as "acceptor") are now widespread. In pressure-sensitive copying paper at least one of the color former and the acceptor is contained in microcapsules so as to be isolated from the other and they become into contact with each other by rupturing such microcapsules to develop a color. In a most typical type of pressure-sensitive copying paper minute oil droplets in which the color former is dispersed or dissolved are encapsulated and coated onto papers.
Microcapsules also find their usefulness in other fields such as adhesives, fertilizers, pharmaceuticals, foods and cosmetics, etc.
There are known various methods for making microcapsules useful for the above mentioned purpose. Among them there are included the "complex coacervation" method, e.g., as disclosed in U.S. Pat. No. 2,800,457, the "simple coacervation" method, e.g., as disclosed in U.S. Pat. No. 2,800,458, the "interfacial polycondensation" method e.g., as disclosed in British Patent Specification No. 950,443 the "in situ polymerization" method e.g., as disclosed in British Patent Specification No. 989,264, the "solidifying-in-liquid" method and the "drying-in-liquid" method.
The microcapsules should have a good impermeability so that the core material which is usually in the form of liquid particles can be prevented from diffusing outside through the capsule wall. The diffusion of the core material in the microcapsules is apt to occur when the environment conditions such as temperature and moisture are changed. Heretofore, various attempts have been made to improve the impermeability of the microcapsules.
The microcapsules having hydrophobic particles as the core material are usually produced through the utilization of the technique of phase separation in an aqueous medium from the viewpoint of its economical advantage and easy working. However, the microcapsules thus produced are not good in the water resistance owing to the property of material used. Some attempts have been made to improve the impermeability of the capsule wall of the microcapsules produced. For example, there have been proposed the utilization of a cross linking agent such as formaldehyde as disclosed in U.S. Pat. No. 2,800,457, the utilization of chemically reactive additives such as phenol compounds and urea as disclosed in Japanese Patent Publications No. 30,030 of 1975, 34,513 of 1975 and No. 34,514 of 1975 and the utilization of polycondensationable prepolymers as the capsule wall forming material as disclosed in Japanese Patent Publications No. 12,518 of 1963, No. 4,717 of 1973 and No. 13,456 of 1974. The proposed techniques are disadvantageous because they require either use of specially selected additives or additional and complicated steps in addition to the fact that the impermeability, especially, the water resistance is not satisfactorily improved.
The water content in the core material and the capsule wall forming material of microcapsules has a great influence on the porosity of the capsule wall which result in decreasing the impermeability. An attempt was made to dehydrate the water contained in the microcapsules by utilizing as a drying agent a water-miscible volatile organic solvent or hygroscopic glycol for improving the impermeability of the capsule wall of the microcapsules, as disclosed in Japanese Patent Publication No. 21,206 of 1971. This technique involves such disadvantages that the core material of the microcapsules is polluted by the drying agent used, that isolation of microcapsules is required before the dehydration step and that it is an expensive way.
The principal object of the invention is to provide a novel and improved method for making microcapsules having a highly improved impermeability.
Another object of the invention is to provide a novel and improved method for making microcapsules, each having a highly condensed and homogeneous capsule wall.
A further object of the invention to provide a novel method of making impermeable microcapsules in a simple and effective manner.